Method and arrangement for protecting a chip and checking its authenticity

ABSTRACT

The semiconductor device has a security coating with embedded magnetic particles and magnetoresistive sensors. This renders possible a measurement of the impedance of security elements defined by magnetoresistive sensors and security coating. If initial values of the impedance arc stored, actual values can be compared therewith to see if the device has not been electrically probed or modified. Such a comparison can be used to check the authenticity of the device.

The invention relates to a semiconductor device provided with a circuit,a security layer that covers the circuit and a security elementcomprising a local area of the security layer and a sensor.

The invention also relates to a carrier provided with a semiconductordevice and a card reader.

The invention further relates to a method of initializing and a methodof checking the authenticity of the semiconductor device.

Such a semiconductor device and such a carrier are known from EP-A300864. The security element of the known device is a capacitor with asits sensor two capacitor electrodes that are coupled capacitivelytogether by the security layer. The device comprises a plurality ofsecurity elements by preference. On checking the authenticity of thedevice, a measured voltage is compared with a calculated referencevoltage. If there is a difference, the authenticity is not recognized.The carrier on which the device is present is a smartcard.

It is a disadvantage of the known device that the security elements canbe circumvented. The security elements may be replaced by otherstructures with the same capacitance which leave the underlying circuitfree. Furthermore, the removal of the security layer and the electrodescannot be detected if the electrodes and the security layer arereapplied afterwards. Such removal is done in order to look at, to probeelectrically, and/or to modify the circuit.

It is therefore a first object of the invention to provide asemiconductor device of the kind mentioned in the opening paragraph, ofwhich removal of the security layer can be detected afterwards.

It is a second object of the invention to provide a carrier with animproved detection of hacking.

The first object is realized in that:

-   -   the security layer comprises embedded magnetic particles, and    -   the sensor is a magnetic sensor capable of measuring a magnetic        property of the security layer.

The second object is realized in that the carrier comprises thesemiconductor device of the invention.

The two features of the invention—the embedded magnetic particles andthe sensor to measure a magnetic property thereof—form in combination avery good system of protection of any semiconductor device. The essencehereof is furthermore that both the sensor and the embedded magneticparticles are implemented on the chip; this also means that the mutualposition does not change. Hence, any uncertainty in the measurement ofthe magnetic property is considerably reduced. Furthermore, themeasurement can be completely hidden for any user or hacker. If thevalues of the magnetic property are stored on the chip itself, thereneed not to be any communication to the outside world. Furthermore, anymagnetic property can be converted easily into values that can betransferred to an outside reader using standard protocols. Possiblemagnetic sensors include magnetoresistive sensors, but also all kinds ofinductors. It is for the ease of signal processing preferred that thesensor can convert the magnetic signal into an electrical or digitalsignal. These two features will now be discussed in extenso one afterthe other.

The use of magnetic particles has the advantage that they aresubstantially inert and that its properties are stable. Furthermore, itis not or hardly possible to provide a security layer with the samemagnetic properties after removal of the original security layer. Aremoval of the security layer can be detected in that at aninitialization an actual value is compared with an initial value thathas been stored in a memory as a reference value. The memory may bepresent outside the semiconductor device. This has the advantage thatone and the same value is available in two locations in thesemiconductor device, and that communication with an external, centraldatabase device is not necessary to check the authenticity.Alternatively, the memory can be present external to the semiconductordevice. This has the advantage that it is not possible to modify thememory and the security layer such that both the reference value and theactual value are different from the original values but neverthelessequal. Preferably, a plurality of security elements is present.

It is preferred that the embedded magnetic particles are inhomogeneouslydistributed over the circuit. The inhomogeneous distribution of theparticles gives the security element an impedance that is specific andunpredictable. The inhomogeneous distribution of magnetic particles inthe security layer over the circuit can be realized in various ways. Ifthe layer is prepared from a particle suspension containing a sol-gelprecursor, an inhomogeneity in the distribution of particles willnaturally be present. This inhomogeneity can be further enhanced byvariation of the suspension parameters, for example by deliberatelycreating an unstable suspension. Another possibility is by depositionaccording to a desired pattern. It is advantageous that the suspensioncomprises a sol-gel precursor, such as a precursor for silica, titania,zirconia, or aluminophosphates. The inhomogeneity may be of chemicalnature—for example chemically different magnetic particles or differentcompositions of magnetic particles—and of physical nature—for exampledifferent particle sizes, or otherwise.

It is furthermore advantageous that, the inhomogeneous distribution isprovided by the addition of non-magnetic particles. This has the resultthat not only the lateral positions of the magnetic particles, but alsoor even mainly the vertical positions of the magnetic particles varyover the circuit. As will be understood, the terms ‘vertical’ and‘lateral’ are used in this context with respect to a plane of referenceparallel to the security layer.

The magnetic particles may be any kind of magnetic particles, such asferromagnetic-, and ferrimagnetic particles. Ferrite particles, such asBaFe₁₂O₁₉, could be used. If the nature of the precursor suspension isincompatible with the magnetic particles, the magnetic particles can beprotected by encapsulation, for example in SiO₂ or in a polymer.

As is known to those skilled in the art, magnetic particles can besubdivided on the basis of their hardness. A parameter for this hardnessis the strength of the coercive field H_(c). A second parametercharacterizing the hardness of a magnetic material is the ratio R of theremanent magnetization M_(r) to the saturation magnetization M_(sat).The remanent magnetization is defined as the magnetization at a zeroexternal field, obtained after a magnetic saturation step. Softmagneticmaterials that are suitable for transformers and inductors have an H_(c)value that is small in an absolute sense and have R<<1. Magneticmaterials with a higher H_(c) and a large R are used for magneticrecording or even as permanent magnets—together also referred to ashardmagnetic materials. Both soft- and hardmagnetic materials can beused in the device of the invention, however in different embodiments.

In a first embodiment, magnetic particles of a softmagnetic material areused, which particles have a diameter on a submicron scale, andpreferably of less than 100 nanometers. Such particles are known assuperparamagnetic particles and are ferromagnetic and ferrimagneticparticles which are so small that, in the absence of an external field,their magnetization fluctuates on a time scale that is much shorter thanthe time period during which a magnetization measurement is carried out.Preferably, a plurality of these particles is provided in an inert,micrometer-sized matrix, and is as such present in the security coating.Such a matrix with superparamagnetic particles is commercially availableand known as a microbead. The softmagnetic material is, for examplemagnetite or a cubic ternary ferrite. With such materials a shortmeasurement time of less than 1 Second, preferably of the order of 0.001to 0.1 Second, can be realized. This is due to the small response timeto the application of a magnetic field of superparamagneticnanoparticles made of these materials. The response time is given by anArrhenius expression, according to which the response time is anexponential function of the product of the magnetic anisotropy energydensity and the particle volume.

The advantage of the superparamagnetic particles is that theirmagnetization M fluctuates on a time scale that is much shorter than thetime period in between of two measurements. As a consequence, theresultant magnetization field H_(//,Sx) parallel to the security layerand at a specific magnetoresistive sensor S_(x) can be assumed to bezero at the beginning of a measurement. Upon application of an externalmagnetic field, a magnetization is induced in the direction parallel tothis external magnetic field. This magnetization induces a magneticdipole field around the particles, which has a significant componentperpendicular to the externally applied magnetic field and substantiallyparallel to the security layer. The resultant magnetization fieldH_(//,Sx) will be different and will induce changes in the resistivityof the magnetoresistive sensors. The magnitude of the resultantmagnetization field H_(//,Sx) will be dependent on the amount ofparticles, and on the distance and location with respect to themagnetoresistive sensor.

In a second embodiment, particles of a hard-magneticmaterial—hard-magnetic particles—are used. The hard-magnetic particlesmay be of any kind or size, and preferably have an average diameterranging from 0.1 to 3 microns. This average diameter is preferably muchsmaller than the thickness of the security layer, which may be as thickas 10 microns or more.

Two sub-classes of hard-magnetic particles are distinguished. If thecoercive field of the particles is much larger than the maximum fieldthat can be allowed to be applied to the chip during its lifetime, themagnetization direction of each of the particles will be permanentlyfixed after magnetizing them once during the fabrication process. Themeasurement of the value of the security elements does then comprise thestep of measuring the resistance of the sensor, without the applicationof an external magnetic field. This value is directly compared with thereference value obtained after the initialization step.

Within the second sub-class, an external magnetic field must be appliedin order to induce the magnetization. Within this second sub-class, thecoercive field of the particles is smaller than or approximately equalto the maximum field that can be allowed to be applied during thelifetime of the chip. The application of a field larger than thecoercive field changes the magnetic state of the particles. Theparticles must therefore be brought into a reference state, in order toremove influences of any uncontrolled previously applied externalmagnetic field. A suitable example of a preliminary treatment isdegaussing. In this treatment, generally applied in cathode ray tubes,an alternating magnetic field is applied. The strength of this field isinitially equal to or larger than a saturation field of the hardmagneticparticles, but is reduced at every alternation to end at a standardvalue, in general a zero field.

The measurement of the value of the security elements with hardmagneticparticles within this second sub-class can be the same as for theparticles of a softmagnetic material. This will comprise the steps ofmeasuring a resistance of the sensor at a reference state in a zeroapplied field, applying an external field in a direction substantiallyperpendicular to the plane of the security layer, said external fieldhaving a strength of at least the saturation magnetic field, andmeasuring the resistance again. The second measurement is preferablystarted after the resultant magnetization field H_(//,Sx) has reachedits saturation value and stopped before the external field is switchedoff.

Alternatively, the external field may be zero or of a strength below thesaturation magnetic field of the hardmagnetic particles, which has theadvantage that the level of security is enhanced and that theapplication of the external field has no or a weaker direct influence onthe resistance of the sensor. In a phase prior to the measurement, thefield may further be applied in a degaussing manner to end up aroundsaid bias value below the saturation magnetic field. Subsequently, ameasurement is carried out. A special case is that in which themeasurement is carried out at zero field.

It is possible to create a well-defined remanent state of the magneticparticles that is different from that in the reference state by makinguse of a variation in time of the external field prior to themeasurement that is different than that used for obtaining the referencestate. The resultant resistivity that is measured is thus not onlydependent on the size and specific distribution of the magneticparticles in the security layer, but also on their detailed hystereticmagnetic response to a time-dependent magnetic field. This enhances thelevel of security for two reasons. Firstly, a prediction of the remanentmagnetization of a particle after a degaussing procedure on the basis ofa measurement of only the volume and of the full hysteresis loops isimpossible in practice, because the so-called inner hysteresis loopsdepend on detailed internal magnetic states of the particle that are notprobed when measuring a full magnetization loop, and because such innerloops are already strongly modified by weak and random variations of theparticle properties that cannot be detected in practice. Secondly, thesensor responses obtained after many different degaussing procedures canbe compared to those obtained in a corresponding way in theinitialization procedure. Such responses are very specific and are to beconsidered as a ‘magnetic signature’. The degaussing procedures can bevaried in length and character, as will be clear to those skilled in theart. Also, use can be made of the full time dependence of the response.

In a further embodiment, superparamagnetic particles, or a mixture ofsuch particles, are chosen such that its or their relaxation time iscomparable to the measurement time. As a consequence, the timedependence of the resistivity can be used in addition to the absolutevalue of the resistivity. This time dependence of the resistivity can bemeasured by the magnetoresistive sensors after the application ofa—sudden—fixed magnetic field.

As explained above, various magnetic sensors can be used includingmagnetoresistive sensors and inductors.

In one embodiment, the magnetic sensor is an inductor. It has been foundthat the magnetic property can be well measured in a stable manner usingnothing more than an inductor as magnetic sensor. In this embodiment, itis suitable to use ferromagnetic particles, such as ferrites, andparticularly but not exclusively magnesiumzincferrites. It is therewithpreferred that these ferromagnetic particles have a high magneticpermeability, for instance μ≧1000. This leads to a reduced sensitivityfor an external magnetic field, but a high sensitivity for theinductors. It is therewith preferred to measure the magnetic property ofthe ferromagnetic particles at higher frequencies, for instance in theMHz range. Ferrite particles have the advantage that they are availablein any desired magnetic permeability and in any diameter, and that theyare completely stable.

The inductors are preferably embodied in the upper layer of theinterconnect structure or even on top of any passivation layer, ifpresent. They may have any suitable shape, which depends on the kind ofmagnetic particles used, and therewith the effect to be measured, aswell as one the available space for one sensor. Principally, there areat least two kinds of shapes for the turns of the inductor. The oneshape uses rectangular turns. This has the advantage that the turns canbe effectively hidden in the interconnect structure, for instance inthat the turn is interrupted in one plane, but continued in anunderlying plane and connected thereto with vertical interconnects. Theother shapes uses circular or oval windings. This has the advantage oflimited space and optimal sensitivity. The sensor can be provided withonly one turn, for instance with a diameter of 1-200 μm, preferablyabout 10-20 μm. It can further be provided with a spirally shapedwinding. A second inductor may be provided concentrically in saidspirally shaped winding. A further structure with appears very suitableis the so-called C-pad structure. In this structure the core of theinductor is formed by a bond pad, or vertical interconnect area.

In another preferred embodiment, the magnetic sensor is amagnetoresistive sensor capable of converting of the magnetic propertiesinto an impedance value. With the magnetoresistive sensor, themagnetization that results from the distribution of the particles istransformed into impedance values. Thus the impedance can be measuredon-line. This has the advantage that any actual value of the impedancecan be further processed, digitized, and stored in an easy manner whichis known in principle. In the context of this application, the term‘impedance’ relates to the impedance as measured in the magnetoresistivesensor. This impedance is in fact an impedance induced by instantaneouschanges in the magnetization in the security element. These changes canbe provided by changing the magnitude of an external magnetic field, andespecially by switching such an external magnetic field on and/or off.The impedance of the magnetoresistive sensor is generally obtained asthe difference between the sensor voltages in the reference state and ina magnetic field, which difference is divided by the sensor current usedin the magnetoresistive sensor. The voltages are preferably, thevoltages of a Wheatstone bridge present in the sensor.

In an advantageous embodiment, the first security element comprises aWheatstone-bridge having a first pair of magnetoresistive sensors and asecond pair of sensors, the sensors of which first and second pair areprovided with substantially the same resistance versus magnetic fieldcharacteristic. Said characteristic is implemented through the physicaland the magnetic structure, for example the sensors have the same sizeand contain the same material, and the pinned layer is pinned in thesame direction in all the sensors of the bridge. The use of a Wheatstonebridge increases the sensitivity of the security element for variationsin the impedance because it is not the impedance itself, but adifference in impedance between the first and the secondmagnetoresistive sensors of the first pair—and optionally the secondpair—that is measured. Besides, with a Wheatstone bridge the measurementis independent of temperature changes and compensates for a constantbackground field. This Wheatstone bridge is known per se to thoseskilled in the art of magnetoresistive sensors. The term Wheatstonebridge is understood to include, in the context of this application, aso-called half Wheatstone bridge comprising a first pair ofmagnetoresistive sensors and a second pair of identical non-magneticelements; a full Wheatstone bridge including a first and a second pairof magnetoresistive sensors; and any modification of a Wheatstonebridge. The magnetoresistive sensors may be of various types such askGMR, TMR, and AMR and are known per se. Besides a standardmagnetoresistive sensor as described with reference to the drawings,more complex sensors may be used. Examples thereof are spin valves withdusting layers, specular spin valves, spin valves with artificialantiferromagnets as pinned layers. If there is a passivation layer underthe security layer, the magnetoresistive sensors may be present oneither side of this passivation layer.

In a further embodiment, the security element has a construction whereinthe magnetoresistive sensors having an axis of sensitivity substantiallyparallel to the security layer are shaped as stripes that have a lengthin a direction substantially perpendicular to the axis of sensitivity.The magnetoresistive sensors of this embodiment are robust in the sensethat a deviation of the magnetic field from the direction perpendicularto the security layer is not harmful. Generally such a deviation isharmful if it saturates the sensor.

Preferably, the passivation structure comprises a plurality of securityelements. These elements may all be security elements comprising atleast one magnetoresistive sensor. However, it may equally well be thatvarious types of security elements are present. Other types of securityelements include capacitors, resistors, inductors, and combinationsthereof, wherein the passivation structure comprises a layer with avarying dielectric constant laterally across the circuit.

As will be explained in more detail below, the impedance measured in thesecurity element must be converted into a signal that can be stored in amemory, either inside the semiconductor device or in any reader ordatabase connected to the reader. To this end, conversion means arepresent to convert an output voltage from the-first security elementinto an actual value of the first impedance. The conversion means may beof well-known nature, such as an A/D-converter or any circuit based on acomparison with a predetermined clock-frequency.

The carrier of the invention may be a smartcard, a record carrier suchas an optical disc, or a security paper such as a banknote.

It is a third object of the invention to provide a card reader withwhich the authenticity of the semiconductor device of the invention canbe checked.

The third object is realized in a card reader suitable for a card with asemiconductor device of the invention, in which card readermagnetization means are present in order to generate an externalmagnetic field that will induce a magnetization in the magneticparticles substantially perpendicular to the security layer. Theexternal magnetic field to be generated preferably has a strength of theorder of 10 to 100 kA/m. Examples of magnetization means include a coiland a permanent magnet. If a coil is used, it may be provided with acore, for example of ferrite material. Furthermore, a number of coils ormagnets that are placed in parallel to each other and are electricallyconnected in series may be used. Such a construction is found to beadvantageous in that a field in substantially one direction isgenerated. A preferred number is two if a field in one direction isdesired. If a field in three directions is desired, the preferred numberis six. The actual card reading part of the card reader is preferablypresent in between the coils or magnets of the magnetization means.

Preferably, a reference sensor is present in de card reader in order tomeasure the external magnetic field. With said measurement the magneticfield can be calibrated. Furthermore, the card reader may containheating means, such as an infrared lamp or another local heat source, orthe provision of a flow of fluid or gas at a specified temperature. Athermometer may be present as well.

In a further embodiment, the coil of the card reader is part of adegaussing circuit. Such a degaussing circuit is known per se from theart of cathode ray tubes. It may be used to provide an adequatemagnetization of permanent magnetic particles, such that any priorexisting magnetization becomes irrelevant. A preferred example of adegaussing circuit comprises a dual PTC thermistor, and a shuntcapacitor parallel to the coil to prevent disturbances.

It is a fourth object to provide a method of initializing thesemiconductor device of the invention.

It is a fifth object to provide a method of checking the authenticity ofthe semiconductor device of the invention.

The fourth object is realized in a method of initializing thesemiconductor device of the invention, in that it comprises the stepsof:

-   -   determining an initial actual value of the impedance of the        security element, and    -   storing the initial actual value as the reference value in a        memory.

The fifth object is realized in a method of checking the authenticity ofthe semiconductor device of the invention, which device has beeninitialized, comprising the steps of:

-   -   determining an actual value of the impedance of the security        element,    -   reading the reference value from the memory,    -   comparing the actual value and the reference value, and    -   recognizing the authenticity of the semiconductor device only if        the difference between the actual value and the reference value        is smaller than a predefined threshold value.

The method of initializing the semiconductor device is necessary,because before the initialization no actual value of the impedance ofthe security element is known. The method of checking the authenticityhas the advantage that both the actual value and the reference value areavailable and can be compared. The actual value is available andphysically fixed in the semiconductor device. The reference value may beavailable in the semiconductor device, but is alternatively available ina central database device to which the card reader has access, or whichis incorporated in the card reader. The reference value could also bepresent both in the semiconductor device and in the central databasedevice. It will be understood that the method can be repeated if aplurality of security elements is present.

The predefined threshold value is generally very small, for examplepreferably below 5% of the reference value, and is to be defined inorder -to correct uncertainties of measurements or influences oftemperature and other external conditions. It is noted that under normalconditions there will be a plurality of security elements, each withtheir own impedances. It may thus be expected that all impedances, or atleast a proportion of them, must be compared with the correspondingreference values before the authenticity of the semiconductor device canbe recognized completely.

If the reference value is stored in a memory of the central databasedevice, the method of checking the authenticity can be interpreted as amethod of identifying the semiconductor device as well; for example,instead of checking whether the actual value is equal to the referencevalue belonging to an already known identity of the semiconductordevice, the actual value can be used to find a corresponding referencevalue in the database, and thus the identity of the semiconductordevice. The use of the reference values in conjunction with a centraldatabase device is generally referred to as a unique chip identifiercode.

In a preferred embodiment, the step of determining the actual valuecomprises the steps of:

-   -   measuring an off-state value at a standard external magnetic        field;    -   generating an external magnetic field to induce a magnetization        in the magnetic particles substantially perpendicular to the        security layer;    -   measuring an on-state value before the external magnetic field        is switched off,    -   determining an actual value of the impedance as the difference        between the on-state value and the off-state value,

As was explained above, only the magnetic particles whose magnetizationcan be permanently fixed can be measured directly. For other magneticparticles it is necessary to apply an external magnetic field beforemeasuring. This external field is preferably generated in the cardreader. In order to have a calibrated actual value, it is measured asthe difference between an off-state value at a standard, preferably zeroexternal field, and an on-state value at the external magnetic field.

If the magnetic particles or at least a proportion thereof contain ahard-magnetic material, a preliminary treatment is necessary to removeany existing magnetization in the magnetic particles in the directionsubstantially perpendicular to the security layer. Such a preliminarytreatment may be a degaussing treatment, such as described above in moredetail.

If the magnetic particles or at least a proportion thereof contain asoft-magnetic material, a relaxation measurement maybe performed,comprising the steps of:

-   -   generating an external magnetic field to induce a magnetization        in the magnetic particles substantially perpendicular to the        security layer;    -   measuring a first and a second value before the particles of the        softmagnetic particles are relaxed to their saturation        magnetization, and    -   determining the actual value of the impedance of the security        element as the difference between the first and the second        value.

This relaxation measurement offers a specific response. The number ofvalues to be measured depends on the relaxation time of thesoft-magnetic material, which is known per se. The actual value isdetermined as the difference between the second and the first value inorder to correct for drift effects. If a large number of values ismeasured, the difference can be calculated between the measured valueand the first value, or between consecutive values. The measurement canbe optimized in that, after measurement of the first and the secondvalue, an external magnetic field is generated in the opposite directionand further values are measured.

These and other aspects of the semiconductor device and the methods ofinitializing it and checking its authenticity according to the inventionwill be further explained with reference to the drawings, in which:

FIG. 1 is a diagrammatical cross-section of the semiconductor device;

FIG. 2 is a diagrammatical cross-section of a security element in thesemiconductor device;

FIG. 3A is a diagrammatical plan view of the security element;

FIG. 3B is a circuit diagram corresponding to the security element shownin FIG. 3A

FIGS. 4A-C show graphs of the applied field, the magnetization, and themeasured voltage difference as a function of time for the embodimentwith magnetic particles of superparamagnetic material;

FIGS. 5A-C show graphs of the applied field, the magnetization and themeasured voltage difference as a function of time for the embodimentwith magnetic particles of hard-magnetic material, wherein measurementtakes place at the saturation field;

FIGS. 6A-C show graphs of the applied field, the magnetization, and themeasured voltage difference as -a function of time for the embodimentwith magnetic particles of hard-magnetic material, wherein measurementtakes place at a field of less than the saturation field; and

FIG. 7 is a schematic diagram of the semiconductor device.

The Figures are schematically drawn and not true to scale, and equalreference numbers in different Figures refer to corresponding elements.It will be clear to those skilled in the art that alternative butequivalent embodiments of the invention are possible without deviationfrom the true inventive concept, and that the scope of the inventionwill be limited by the claims only.

In FIG. 1, the semiconductor device 11 has a substrate 31 of silicon,having a—first—side 32. On this side 32, the device 11 is provided witha first active element 33 and a second active element 43. These activeelements 33, 43 are bipolar transistors with emitter regions 34, 44;base regions 35, 45 and collector regions 36,46 in this example. Saidregions 34-36, 44-46 are provided in a first layer 37, which is coveredwith a patterned insulating layer 38 of silicon oxide. The insulatinglayer 38 is patterned such that it has contact windows at the emitterregions 34, 44 and the base regions 35, 45. As known to those skilled inthe art, field effect transistors may be present instead of or besidesthe bipolar transistor. As is further known to those skilled in the art,other elements such as capacitors, resistors, and diodes may beintegrated in the semiconductor device 11. The active elements areinterconnected so as to form a circuit.

At these contact windows in the insulating layer 38, the said regionsare connected to interconnects 39, 40, 41, 42. The interconnects in thisembodiment extend at a first level and a second level. As is generallyknown, the interconnect structure may comprise more levels. A barrierlayer not shown is generally present between the interconnects and theactive elements. The interconnects 39, 40,41,42 are manufactured, forexample, in Al or in Cu, in a known manner and are covered and mutuallyinsulated by dielectric layers 47 that preferably have a low dielectricconstant. Additionally, present barrier layers are not shown. Athird-level interconnect 28 is present to connect the security element12, comprising a first and a second magnetoresistive sensor 121, 122 anda local area of a passivation structure 50.

This passivation structure 50 in this embodiment comprises a passivatinglayer 52 of Si_(x)N_(y) with a thickness of 0.60 μm. Under thepassivating layer 52 a further layer of phosphosilicate glass may bepresent. The passivation structure further comprises a security layer 53of aluminophosphate with a thickness of 2-10 μm in which magneticparticles are embedded. TiO₂ and TiN particles are also present in orderto stabilize the security layer 53 and to decrease the transparency ofthe layer. A planarizing not shown layer may be present below thepassivating layer 52. The security layer 53 was applied by spincoating acomposition of 15% by weight of monoaluminumphosphate, 20-50% by weightof particles in water and subsequent drying at about 100-150° C.Alternatively, it may be applied by spraycoating a composition of 5-10%by weight of monoaluminumphosphate. After drying, the layer is annealedat 400-500° C. to allow condensation, whereupon a transition from thefluid to the solid phase takes place. On the security layer 52 an epoxymaterial is present as a package 54. The security layer 53 may bepatterned, so as to facilitate sawing of the wafer into separate dies,and to define contact pads for connection to a PCB, for example.

The sensors 121, 122 are at a mutual distance of about 1 micrometer.Their functioning will be explained in more detail with reference to theFIGS. 2 and 3. The sensors 121,122 may be present at greater mutualdistances. If, however, the distance is smaller than 2 microns, themeasurement is improved. This is due to the fact that magnetic particlesthat are present in between the sensors will induce magnetization inopposite directions in the sensors, and thus different changes in theimpedance.

FIG. 2 is a diagrammatical cross-section of a detail of the securityelement 12. The magnetoresistive sensors 121, 122 each comprise a stackof four main layers: a pinning layer 61, a pinned layer 62, a spacerlayer 63, and a free layer 64. The pinning layer 61 is anantiferromagnet, in this case a 10 nm thick Ir₂₀Mn₈₀ layer. It may beinsulated from the underlying structure through one or more bufferlayers, such as 3 nm thick layers of Ta and/or Ni₈₀Fe₂₀. The pinnedlayers 62—in this case 6 nm Co—has a magnetization that is not variableowing to the influence of the pinning layers 61. It is preferred thatthe magnetization of the pinned layers 62 of the magnetoresistivesensors 121, 122 are in parallel directions. The output voltage of thebridge is then not sensitive to a uniform external magnetic field. Thespacer layer 63 comprises a conductive material such as Cu with athickness of 3 nm in the preferred case of a GMR sensor. In the case ofa TMR sensor, an insulating material such as Al₂O₃ with a thickness of 1nm is applied. The free layer 64 comprises a soft-magnetic material suchas Ni₈₀Fe₂₀ with a thickness of about 6 nm.

FIG. 2 shows the situation that there are three superparamagneticparticles present in the security layer 53 near to the magnetoresistivesensors 121,122, of which the axis of sensitivity is parallel to thedirection of the magnetization in the pinned magnetic layers 62 (thex-axis). The particles are of different size and are present atdifferent distances and angles with respect to the sensors 121,122.After application of a magnetic field that is oriented perpendicularlyto the plane, with a time dependence that will be further explained withreference to FIG. 4, a perpendicular magnetization will be induced inthe particles. This results in a dipolar field around the particle, asindicated schematically in the Figure by magnetic field lines. Thedipolar field from the magnetic particles A, B, and C will exert amagnetic torque on the magnetization of the free layers 64 of thesensors 121, 122, which in the absence of the dipolar fields areoriented substantially parallel to the y-direction (i.e. the directionperpendicular to the plane of drawing). The torque depends on thedistance in the x and z (perpendicular to the layer plane) directionsbetween the particles A, B, C and the sensor, and is proportional to thestrength of the magnetization of the particles A,B,C. As a consequence,rotations of the magnetization are induced in the free layers 64. Thedirections and sizes of these magnetization rotations are determined bythe directions and sizes of the effective (layer-averaged) x-componentsof the magnetic fields induced by the magnetic particles A,B,C. Themagnitude of these fields, at distinct positions in the sensor plane, issymbolized in the Figure by the lengths of the arrows. As a consequence,there is a net magnetization rotation to the right (i.e. in the positivex-direction) in the first sensor 121 and a net magnetization rotation tothe left in the second sensor 122. The net x-component of themagnetization of the free layer 64 in the first sensor 121 is thusgreater than that of the free layer 64 in the second sensor 122. Theresistance of the magnetoresistive sensors 121,122 depends on the anglebetween the magnetization directions of the pinned and the free layer62,64. As a consequence, the resistance of the sensor 121 is decreasedcompared to average, whereas the resistance of the sensor 122 isincreased.

FIG. 3 a is a diagrammatic plan view of the security element 12. FIG. 3b shows an equivalent circuit diagram. The security element 12 is aWheatstone bridge. The parts 123, 124 may be either equal non-magneticresistors or magnetoresistive sensors, preferably of the same type asthe sensors 121, 122. Although preferred, it is not necessary that theparts 123,124 have the same physical dimensions as the sensors 121,122.The security element 12 comprises, besides the parts 121-124 and thesecurity layer (not shown), electrodes 131-134. The first electrode 131is a current input, the second and the third electrode 132,133 aremutually connected via a voltage measurement. Conversion means arepresent to convert an output current or voltage from the securityelement into an actual value of the impedance. The conversion means willbe further explained with reference to FIG. 7. The fourth electrode 134is a current output. It is observed that the shape of the Wheatstonebridge as shown in FIG. 3 a is not essential for the embodiment. This isdue to the randomness of the distribution of the magnetic particles.

FIGS. 4, 5, and 6 show graphs of the applied field, the magnetization,and the measured voltage difference for three embodiments of theinvention. FIG. 4 relates to the embodiment with superparamagneticparticles. FIGS. 5 and 6 relate to the embodiment with hardmagneticparticles of which a reference state is defined prior to themeasurement. In FIG. 5 shows a measurement at the saturation field, andFIG. 6 a measurement at a degaussed field of less than the saturationfield.

When superparamagnetic particles are used, the magnetization of theparticles is zero before the application of an external field. Thereforeone can perform an a off-state measurement of the output voltage of theWheatstone bridge immediately. This measurement will begin at t_(R,B)and end at t_(R,E). Then the external magnetic field H_(app) is appliedat t₀. This will lead to an increase of the magnetization of theparticles M to their saturation value M_(sat), on a time scale that isdetermined by the relaxation time or the relaxation time distribution ofthe particles. The magnetization then remains stable as long as thefield H_(app) is present, and a measurement of the voltage difference ΔVcan be made. This on-state measurement will begin at t_(B) and end att_(E). Finally at t₁, the external magnetic field H_(app) will dropswitched off and the magnetization M and the voltage difference ΔV willreduce to their reference values. The actual value is determined as thedifference between the on-state measurement and the off-statemeasurement. Alternatively, the measurement of the voltage differencemay be carried out as a function of time. This is of most interest ifthe relaxation time is of the order of the time t₁.

When using hard-magnetic particles of which the coercive field issmaller than or of the same order of magnitude as the-maximumuncontrolled external field that is allowed, a pretreatment is necessaryto remove any remanent magnetization. A preferred method for this is adegaussing treatment. In such a degaussing treatment, as shown in FIG.5A, an oscillatory external magnetic field is applied with alternatingdirections and decreasing maximum strengths. Then the off-statemeasurement is made, the external field H_(app) is applied to t₀, andthe on-state measurement is performed from t_(B,1) to t_(E,1). Theactual value is again determined as the difference between the on-stateand the off-state values. After switching off of the field H_(app) att₁, a remanent magnetization will usually still in be present. Thisremanent magnetization, which is a materials property, can be used foran additional measurement, from t_(B,2) to t_(E,2).

Alternatively, the measurement may be preceded by a degaussing treatmentin a specified manner that is different from that used for obtaining thereference state, for example degaussing around a certain bias field, asis shown in FIG. 6. The subsequent measurement may take place at afinite field, for example the bias field around which degaussing hastaken place. It may also take place after the external final field hasbeen switched off.

FIG. 7 is a diagram of an embodiment of the semiconductor device 11together with an access device 2. The semiconductor device 11 comprisesvarious means: measuring means 4, memory 7, control means 8, and averification control 9. Furthermore, the semiconductor device comprisesa plurality of security elements 12 as well as a switch 10. The memory 7comprises a plurality of memory elements 7A, 7B, 7C . . . , as well as astorage control 5 and read control 6. The control means 8 and theverification control 9 may be integrated into one function, this being amicroprocessor, or a dedicated circuit. The control means 8 need not bededicated solely to the control of the measuring, storing, and readingof the impedances of the security elements 12, but may control thefunctioning of the complete semiconductor device, including a furthermemory with financial or identity data. The access device 2 is generallya card reader, but may be another device, for example an apparatus withwhich the initialization is done.

This example of a exemplary circuit in the semiconductor device 11functions as follows: a signal is sent from the access device 2 to thesemiconductor device 11 requesting the initialization or authenticitycheck. Values of the impedances of the security elements 12 are measuredvia control means 8, and are sent to conversion means 4 with a frequencydepending on the impedance, and then go via a switch 10 to the memory 7.The conversion means generally comprise an oscillator, a counter, and areference oscillator to provide a clock frequency, or a standard A/Dconverter. The result is a digitized signal, representing the actualvalue of the impedance of the measured security element. It may bepresent in any kind of SI-unit, but also in any device-specific value ifit is not to be compared with any externally measured value. Dependingon the switch 10, the actual value may be stored or provided to theverification control 9. The switch is preferably switchable only once,for example in that it comprises a fuse. It is not excluded, as will beapparent to those skilled in the art, that the switch 10 and the storagecontrol 5 are integrated into one functional unit. The verificationcontrol 9 will compare the actual value and the reference value. If thedifference between the two values is smaller than a predefined thresholdvalue, for example 3%, then a positive signal—stating okay—will be sentto the control means 8. This may be done immediately or after all theactual values have been compared with all reference values, or aftercomparison of a selected number of the actual values have been comparedwith the corresponding reference values. The predefined threshold valuewill be dependent on the precision of the measuring means. It could be10 or 20% as well, especially if the number of security elements islarge, for instance 10 or more. It could be less than 1% as well, whichis partially dependent on the customer's wishes and the state of the artof integrated circuit design.

1. A semiconductor device provided with a circuit, a security layer thatcovers the circuit, a security element comprising a local area of thesecurity layer, and a sensor, characterized in that: the security layercomprises embedded magnetic particles, and the sensor is a magneticsensor, capable of measuring of a magnetic property of the securitylayer.
 2. A semiconductor device as claimed in claim 1, characterized inthat the magnetic sensor is a magnetoresistive sensor, capable ofconverting the magnetic properties into an actual value of theimpedance.
 3. A semiconductor device as claimed in claim 1,characterized in that the embedded magnetic particles are distributedinhomogeneously in the security layer (53) over the circuit.
 4. Asemiconductor device as claimed in claim 1, characterized in that themagnetic particles are superparamagnetic particles embedded inmicrobeads.
 5. A semiconductor device as claimed in claim 1,characterized in that the magnetic particles comprise a hard-magneticmaterial.
 6. A semiconductor device as claimed in claim 2, characterizedin that the magnetoresistive sensors having an axis of sensitivitysubstantially parallel to the security layer are shaped as stripes thathave a length in a direction substantially perpendicular to the axis ofsensitivity.
 7. A semiconductor device as claimed in claim 1, furtherprovided with a memory for storing an initial actual value of theimpedance of the security element as a reference value.
 8. A carrierprovided with a semiconductor device as claimed in claim
 1. 9. A cardreader suitable for a carrier as claimed in claim 8, characterized inthat magnetization means are present in order to generate an externalmagnetic field that will induce a magnetization in the magneticparticles substantially perpendicular to the security layer.
 10. A cardreader as claimed in claim 9, characterized in that a reference sensoris present for measuring the external magnetic field, so that theexternal magnetic field can be calibrated.
 11. A card reader as claimedin claim 9, characterized in that the magnetization means are part of adegaussing circuit.
 12. A method of initializing the semiconductordevice as claimed in claim 1 comprising the steps of: determining aninitial actual value of the impedance of the security element, andstoring the initial actual value as the reference value in a memory inthe semiconductor device or in a central database device located in orconnected to the card reader as claimed in claim
 9. 13. A method ofchecking the authenticity of a semiconductor device as claimed in claim1, the device being initialized by the method of claim 12, comprisingthe steps of determining an actual value of the impedance of thesecurity element, reading the reference value from the memory, comparingthe actual value and the reference value, and recognizing theauthenticity of the semiconductor device only if the difference betweenthe actual value and the reference value is below a predefined thresholdvalue.
 14. A method of initializing or checking as claimed in claim 12,characterized in that the step of determining an actual value comprisesthe steps of: measuring an off-state value at a standard externalmagnetic field; generating an external magnetic field to induce amagnetization in the magnetic particles substantially perpendicular tothe security layer; measuring an on-state value before the externalmagnetic field is switched off, determining the actual value of theimpedance of the security element as the difference between the on-statevalue and the off-state value.
 15. A method of initializing or checkingas claimed in claim 14, characterized in that: at least a proportion ofthe magnetic particles embedded in the security layer of thesemiconductor device comprise a hardmagnetic material; and beforemeasuring of the off-state value, a preliminary treatment is performedin order to remove any existing magnetization in the magnetic particlesin the direction substantially perpendicular to the security layer. 16.A method of checking the authenticity as claimed in claim 15,characterized in that the external magnetic field is generated at astrength below the saturation magnetization field strength of at leastproportion of the magnetic particles.
 17. A method of checking theauthenticity as claimed in claim 15, characterized in that the externalmagnetic field is alternating, and the magnitude of the field decreasesdown to an average bias field below the saturation magnetization fieldof at least a proportion of the magnetic particles.
 18. A method asclaimed in claim 12, characterized in that at least proportion of themagnetic particles embedded in the security layer of the semiconductordevice comprise a soft-magnetic material, and that the step ofdetermining an actual value comprises the steps of: generating anexternal magnetic field to induce a magnetization in the magneticparticles substantially perpendicular to the security layer; measuring afirst and a second value before the soft-magnetic particles are relaxedto their saturation magnetization, determining the actual value of theimpedance of the security element as the difference between the firstand the second value.